1. Field of the Invention
The present invention relates to a memory controller for accessing in a memory interleaving manner to a semiconductor memory section capable of storing multi-bit data in one memory cell, a memory system having the memory controller, and a control method for the memory system.
2. Description of the Related Art
Today, flash memory devices, which are non-volatile memory systems, are widely used as external storage devices for digital cameras or the like serving as hosts, startup memory systems for computer systems or the like because flash memory devices permit rewriting of data and are capable of maintaining data even in powered-off state.
Among such flash memory devices, NAND-type flash memory devices are known as non-volatile memory systems that realize large capacity and low cost. NAND-type flash memory uses electric charge that is injected via insulating film into a charge accumulating layer of memory cells of a semiconductor memory section as digital bit information, measures a difference in threshold voltage of transistors determined by an amount of electric charge, and reads out information.
To accomplish further increased capacity and reduced cost, aggressive research and development of NAND-type flash memory devices that use a multi-level technique for storing data of two or more bits in a single memory cell, so-called a multivalued flash memory device (hereinafter also called “multivalued memory”), has been conducted. For instance, Japanese Patent Application Laid-Open Publication No. 2001-93288 discloses a memory system capable of storing two-bit data by means of four different threshold voltages of one memory cell.
Meanwhile, memory interleaving is known as a technique for increasing access speed of a memory system. Memory interleaving increases data transfer rate by concurrently accessing a number of memory chips in parallel. For instance, Japanese Patent Application Laid-Open Publication No. 2007-334863 discloses a NAND-type flash memory device that accesses two memory chips in an interleaved manner.